Glucose metabolism of fed, starved and toxaemic pregnant sheep

Abstract

Glucose metabolism was studied in fed, starved and toxaemic ewes in the last month of gestation to ascertain causes of hypoglycaemia, and to determine whether the glucose metabolism of sheep susceptible to ovine pregnancy toxaemia (OPT) differs from those not susceptible.

Glucose kinetics were measured with a single injection of [U- ¹⁴C], [6-³H] glucose. Subsequently glucose tolerance tests were performed using 0.4 g glucose/kg liveweight.

Ewes were starved for 10 days to induce OPT, and blood samples were taken from one jugular vein and analysed for packed cell volume, glucose, ketones, free fatty acids, urea, creatinine, inorganic phosphate, total CO₂, total plasma protein and albumin. Glucose turnover was measured by a primed continuous infusion of [U -¹⁴C], [6-³H] glucose when the ewe s became recumbent with OPT (and these were classified as susceptible, S) otherwise at the end of 10 days (and these were classified as non-susceptible, NS). The ewe was slaughtered at the end of the infusion. The caudate lobe of the maternal liver was removed, and perfused with collagenase to prepare hepatocytes for measuring the gluconeogenic potential of the liver.

In another experiment, the effect of live foetuses on maternal glucose metabolism was assessed. Nooses were surgically implanted around the umbilical cords of foetuses in twin-pregnant ewes in the last month of gestation. Ten days after surgery, the ewes were starved for four days. Glucose turnover was measured by continuous infusion of [U-¹⁴C], [6-³H] glucose and then the foetuses were killed by tightening the nooses. Blood samples were taken for 24 hours, to monitor glucose and acetoacetate concentrations. The ewe was then given a second infusion of tracer glucose, and subsequently killed for hepatocyte studies.

A 5-compartment model was proposed for glucose kinetics of fed, twin-pregnant ewes. Data of Sand NS ewes were compared by fitting to the model separately, using the SAAM program.

Ewes susceptible to OPT had significantly higher insulin resistance (2043 µU/ml. min) than NS ewes (1261 µU/ml. min). This criterion could be useful in screening sheep for susceptibility before inducing the disease, and also provides evidence that OPT may be related to a poor ability to maintain glucose homeostasis.

During starvation plasma glucose decreased, while ketones and free fatty acids increased. The final concentrations were related to the state of the lambs rather than OPT symptoms. Indications of renal failure (raised serum creatinine, urea and inorganic phosphate, and lowered total CO₂) were observed in both S and NS ewes. On post-mortem analysis, liver glycogen of S ewes was 10% (2 mg/g) the level of NS ewes (18 mg/g).

The gluconeogenic potential of hepatocytes from S ewes, with the substrates propionate, lactate, alanine, glutamine and glycerol, was significantly lower than that of NS ewes and the rates were not stimulated by glucagon. It is hypothesized that the development of OPT may be related to a lesion in glucose production.

Ewes with dead lambs were often hyperglycaemic (plasma glucose greater than 80 mg/dl) had significantly higher glucose turnover rates (6.90 g/kg/day) than those with live lambs (2.67 g/kg/day), and had significantly higher gluconeogenic potentials, measured on hepatocytes. This suggested that the maternal glucose production was restricted by the presence of live foetuses.

Killing the foetuses in utero raised plasma glucose (from 27 to 99 mg/dl), increased glucose turnover (from 2.39 to 4.82 g/kg/day) and significantly increased hepatocyte glucose production. It is hypothesized that the hypoglycaemia of starved pregnant ewes is due to inhibition of hepatic glucose production by some foetoplacental factor.